Machines that cut hard surfaces such as used in the trenching and mining industries, and that remove hard surfaces of concrete and asphalt prior to repaving a roadway require rotatable tools mounted in a wheel or rotatable drum. The tools have a tapered forward cutting end and axially located behind the cutting end is a cylindrical shank that rotatably fits within a complementary shaped bore of a tool holder. Between the forward cutting end and the shank, the tool has a rearwardly directed annular flange that contacts the forward surface of the tool holder. Force is applied through the rotatable drum or wheel to the tool holder and through the radial flange to the tool to thereby force the tool into the hard surface to be cut. The tools are mounted on the drum at an angle such that rotation of the drum causes rotation of the tools with the annular flange rotating against the planar surface of the tool holder.
To prevent wear to the forward end of the tool holder it has become customary to provide a wear ring that fits around the circumference of the shank of the tool with one surface of the wear ring engaging the annular flange of the tool and the other surface engaging the forward surface of the tool holder. The useful life of a tool used in road planing is at most one day during active use of the machine. It is therefore common to replace all the tools of a road planing machines at least daily. When the tool is replaced, the wear ring is also replaced such that the contact is between the annular flange of a new tool and the forward surface of a new wear ring.
In order for the parts to work properly, it is desirable that the wear ring itself be non-rotatable with respect to the tool holder. There are several methods employed for preventing rotation of the wear ring with respect to the tool holder, one of which is described in my U.S. Pat. No. 7,229,136 B2 issued Jun. 12, 2007, which is incorporated herein by reference. Another structure is shown in Britzke, U.S. Pat. No. 5,931,542.
The tool further has a hardened forward tip that engages the hard surface to be cut. The rotation of the tool causes the forward cutting end of the tool to become worn symmetrically around its longitudinal axis. The rotation of the tool is therefore necessary to sharpen the hardened tip. Where the wear ring fails to rotate properly, the forward cutting end will wear unevenly and the hardened tip will gradually become blunted or develop a flat surface and the useful life of the tool will become shortened.
The frictional force generated between the forward surface of the non-rotatable wear ring and the rearward surface of the rotating annular flange of the tool inhibits rotation of the tool. It is therefore desirable to minimize the resistance forces between the wear ring and the annular flange.
The annular flanges of all tool holders are planar and oriented perpendicular to the longitudinal axis of the tool body. The forward surfaces of existing wear rings are also planar such that two planar surfaces are in uniform contact with each other as the tool rotates.
Not withstanding all the above, tool failure continues to occur as a result of inadequate rotation of the tool within the tool holder. It is therefore desirable to minimize the resistance between the rearward surface of the annular flange and the forward surface of the wear ring wherever possible.